1. Field of the Invention
Generally, the present invention relates to magnetic resonance imaging apparatuses. More specifically, the present invention relates to a technique for damping vibrations of a static magnetic field generator in MRI apparatuses.
2. Description of the Related Art
A typical magnetic resonance imaging apparatus (hereinafter, called “MRI apparatus”) is equipped with a static magnetic field generator. This static magnetic field generator emits a uniform static magnetic field to a test subject, so that he or she is exposed to the static magnetic field. Simultaneously, the MRI apparatus irradiates the test subject with an electromagnetic wave. In response to this, the apparatus detects a magnetic resonance signal emitted from the test subject, and the signal then undergoes an image process. Consequently, the apparatus produces magnetic resonance images which exhibit the physical property of the test subject.
Furthermore, a typical MRI apparatus is provided with a gradient magnetic field generator, which emits a gradient magnetic field. The gradient magnetic field is superimposed on the static magnetic field, so that the magnetic resonance signal is pinpointed.
Such MRI apparatuses have two types: one is a horizontal static magnetic field type and the other is a vertical static magnetic field type.
A horizontal static magnetic field type has a cylindrical static magnetic field generator. A test subject is set into this generator, while the axis of his or her body is aligned with the orientation of the static magnetic field.
A vertical static magnetic field type has a static magnetic field generator, two pieces of which are placed opposite each other vertically. A test subject is set between the pieces. Therefore, the axis of his or her body is perpendicular to the orientation of the static magnetic field.
The vertical static magnetic field type is superior to the horizontal type, in that a test subject does not feel stuffy in the generator and an inspector can access the interior of the generator easily.
The quality of magnetic resonance images is influenced by the intensities of the static magnetic field and the gradient magnetic field. As the magnetic fields are stronger, the quality of the images is enhanced. The static magnetic field generator is made of permanent, resistive or superconducting magnets. Especially when the strong static magnetic field, such as more than 0.4 tesla, is necessary, a static magnetic field generator made of a superconducting magnet is typically used.
The quality of magnetic resonance images also depends on the uniformity of the static magnetic field. As the uniformity is better, the quality of the images is enhanced. The uniformity of the static magnetic field lowers, when the static magnetic field generator vibrates. Thus, the quality of magnetic resonance images is affected by the vibration of the static magnetic field generator. In order to provide a sufficient level of image quality, the uniformity of the static magnetic field needs to be on the order of 100 ppm. For an MRI apparatus of a few meters in height, an allowed vibration level is on the order of several micrometers. Thus, an allowed vibration level is up to one per million of the height of an apparatus.
As described above, such an MRI apparatus produces images of a test subject, while irradiating him or her with an electromagnetic wave by using high-frequency coils. Furthermore, the apparatus feeds a current to its gradient magnetic field generator in sync with the irradiation of the electromagnetic wave, thereby emitting a gradient magnetic field. As a result, a current flows within the static magnetic field. This results in the generation of the Lorentz force, which vibrates the gradient magnetic field generator. In conjunction of this vibration, the static magnetic field generator also vibrates.
Furthermore, as the static magnetic field or gradient magnetic field is stronger, a static magnetic field generator vibrates more powerfully. If one or both of the static magnetic field and gradient magnetic field is strong for the purpose of providing high image quality, the vibrating force produced by the gradient magnetic field generator increases. Consequently, the static magnetic field generator vibrates more powerfully.
Such a vibration may become an important concern for vertical static magnetic field type of MRI apparatuses. A vertical static magnetic field type of MRI apparatus has a structure where the two pieces of a static magnetic field generator are coupled to each other, with being placed opposite one another vertically. This structure is difficult to have a sufficient structural strength, because it is required to take full advantage of the merit that enables a test subject not to feel stuffy or an inspector to access the interior of the apparatus easily. Moreover, the center of gravity of the apparatus is prone to be high. Therefore, the apparatus is more likely to vibrate. As a result, a technique for damping the vibrations becomes important.
A technique for damping vibrations of MRI apparatuses is disclosed by Japanese Unexamined Patent Application Publication 2005-137530 (see page 9 and FIG. 1). A disclosed MRI apparatus is a typical vertical static magnetic field type of static magnetic field generator made of superconducting magnets. Specifically, this MRI apparatus has a structure where a static magnetic field generator is made of superconductive coils with being placed opposite each other vertically across an image pickup space. In addition, each superconductive coil is contained in a cooling container, which is housed in a vacuum container. The cooling containers are coupled through a cooling container connection pipe, thereby constituting a cooling container module (inner chamber system). The vacuum containers are coupled through a vacuum container connection pipe, thereby constituting a vacuum container module (outer chamber system). Moreover, the gradient magnetic field coils, which apply a gradient magnetic field to be superimposed in a static magnetic field, are placed opposite each other across the image pickup space. These coils are supported by the vacuum containers, respectively.
As described above, in such MRI apparatuses, the electromagnetic wave generated upon the production of images involves the vibration of the gradient magnetic field coils. As a result, the vacuum containers supporting the gradient magnetic field coils, that is, the vacuum container module also vibrates. This vibration is considered to be a vibration of a first bending mode, because the lower surface of the lower vacuum container is restricted by a floor. Accordingly, the vibration of the lower vacuum container is weaker than that of the upper vacuum container. The MRI apparatus of JP2005-137530 takes advantage of this property in order to decrease the vibration of the cooling containers or superconductive coils, which is caused by the vibration of the gradient magnetic field coils. To demonstrate, multiple support members are arranged between the lower cooling container and the lower vacuum container. In addition, the cooling container module is supported by the lower vacuum container through the supporting members. Thus, the lower vacuum container that vibrates slightly is configured to support the cooling container module. With this structure, the vibration of the cooling containers, that is, of the superconductive coils can be damped.
It is assumed that a floor where an MRI apparatus is installed vibrates by itself. In this case, the MRI apparatus vibrates due to the vibration transferred from the floor (external vibration), as well as by the self-vibration of the apparatus. In order to prevent the external vibration, the technique disclosed by Japanese Unexamined Patent Application Publication 2004-267397 has been conceived (see page 14 and FIG. 1). The disclosed MRI apparatus is equipped with a supporting mechanism that supports a magnet for generating a static magnetic field on a floor. This supporting mechanism is composed of a position adjustment unit and a damping unit. The position adjustment unit is adapted to adjust the position of the magnets. The damping unit is adapted to damp the vibration of the floor by changing the frequency of the vibration transferred from the floor into a frequency other than a resonance frequency of the magnets. This MRI apparatus can damp the external vibration efficiently. Thus, it is possible to prevent the deterioration of magnetic resonance images, even if any vibration source is present on a floor.
Now, a description will be given, of the types of vibrations. Among vibrations caused due to the inner structure of the MRI apparatus, the following vibrations are considered to affect images:
1) a vibration of a first bending mode, that is, a vibration caused by an apparatus of which lower surface is restricted by a floor;
2) a rocking vibration, that is, a vibration caused by an apparatus of which apparatus bottom surface is free, for example, is supported by a spring, etc; and
3) a vibration of lower surface of an apparatus.
The rocking vibration means continuous rocking of the apparatus due to the vibration of the gradient magnetic field coils. The apparatus seems to rotate taking a floor as a fulcrum. Specifically, it is considered that an apparatus is placed directly on a floor while being in contact with the floor. In this case, if the floor is not flat, then the lower surface of the apparatus may be in point contact with the floor. Thus, the installed apparatus is unstable. Then, the apparatus rocks, as if it rotated taking the contact point as a fulcrum. This vibration is called the “rocking vibration”. The rocking vibration can be ignored on the condition that the static magnetic field is not strong. However, when the intensity of the static magnetic field is 1 tesla or more, the rocking vibration may affect images.
Next, the vibration of lower surface of an apparatus will be described. It is assumed that an apparatus is placed on a floor with being partially supported. In this case, a vibration of a bending mode may occur on portions that are off from the floor. This vibration is called the “the vibration of lower surface of an apparatus”. This vibration may affect images, when the intensity of the static magnetic field exceeds a predetermined level, similar to the rocking vibration.
In order to damp the vibration of MRI apparatuses, JP2005-137530 discloses the mechanism for efficiently damping the vibration of superconductive coils (or a static magnetic field generator) due to the vibration of the first bending mode. This mechanism can be effective to damp the vibration to the extent that images are not deteriorated. This technique adapts a structure where supporting members are arranged between a lower cooling container and a lower vacuum container. Due to those members, the vibration transferring from the vacuum container module to the cooling container module is attenuated. As a result, the vibration of the cooler container and the superconductive coils is damped. However, this structure does not damp the rocking vibration sufficiently. Hence, any other structures are necessary, if the rocking vibration needs to be damped efficiently.
The rocking vibration can occur when the installed apparatus is unstable, as described above. Accordingly, in order to damp the rocking vibration efficiently, the installed apparatus needs to be stable. To attain this object, an adjustment structure for partially supporting an apparatus, such as the support mechanism disclosed by JP2004-267397, is effective. In addition to this structure, an anchor structure for supporting the apparatus by using an anchor is also effective.
However, the above adjustment structure and the anchor structure may cause the cost increase. Furthermore, the working hours for installing an apparatus with the above structure may be prolonged. In addition, the adjustment structure needs an enlarged installation space in the vertical direction. In consideration of the above disadvantages, another structure by which an apparatus can be installed easily and stably is required.
Moreover, the above adjustment structure and the anchor structure may not be sufficient to damp the vibration of the lower surface. The vibration of lower surface of an apparatus can be damped by supporting portions of the lower surface where the loops of the vibration appear. However, the above adjustment structure and anchor structure can support only the surrounding portions of the lower surface. Thus, they fail to damp the vibration of the lower surface efficiently.
Taking the above disadvantages into account, the present invention has been conceived. An object of the present invention is to provide a simple MRI apparatus which can be kept stably installed. An additional object of the present invention is to present an MRI apparatus which is configured to damp the rocking vibration and the vibration of its lower surface efficiently.